Typically, a compressed gas has a high moisture content upon leaving the compressor. This is particularly the case with compressed air. Moisture present in the compressed gas can give rise to problems in the equipment using the compressed gas and is generally undesirable. In certain applications, it is necessary to reduce the moisture content of the compressed gas, before it can be used.
A number of drying systems are known for use in the drying of compressed gas. Most commonly applied are refrigerated dryers, deliquescent dryers and desiccant dryers. Desiccant dryers are known in the art and have been applied in the preparation of dried compressed gas. The desiccant dryers may operate as so-called "single shot" dryers, in which the desiccant is used to dry the gas until it is saturated with moisture, after which it is removed and replaced with fresh desiccant. A preferred desiccant dryer is the so-called "regenerative" dryer, in which the desiccant may be regenerated in situ and reused, without the need for removal and replacement. The regenerative desiccant dryers may employ pressure or vacuum swing techniques to regenerate the desiccant. Alternatively, the desiccant may be regenerated using heat to remove the accumulated moisture from the desiccant.
Heat regenerative desiccant dryers typically operate as follows. Gas to be dried is passed through the bed of desiccant. Moisture in the gas is adsorbed by the desiccant. Once the desiccant is saturated with moisture, the unit is typically taken off-line and the desiccant heated. Heating the desiccant evaporates the moisture, which leaves the dryer as vapor. Once the desiccant is dried, the unit is again placed on-line to continue drying the gas. A variety of schemes for heating the saturated desiccant and regenerating it have been disclosed in the prior art. A number of the prior art systems rely upon heating elements to provide the necessary heat to regenerate the adsorbent. Such systems require an additional external source of power in order to generate the necessary heat. Alternative systems attempt to utilize the heat produced during the compression of the gas. Such systems use the compressed gas itself either directly or indirectly to heat the adsorbent in order to effect regeneration. A number of the systems employ dried compressed gas product in order to carry out the regeneration procedure. Still further prior art systems rely upon the heat generated during the compression of the gas, for example using oil withdrawn or recovered from the compressor as the heating medium.
In the method and apparatus disclosed in U.S. Pat. No. 4,898,599 a gas, typically air, is compressed using an oil flooded screw compressor. The compressed gas is dried by being passed through a bed of moisture adsorbing material or desiccant. In order to regenerate the saturated adsorbent, hot oil from the compressor is recovered from the compressed gas stream leaving the compressor and passed through heat exchange tubes extending through the adsorbent bed. Heat from the oil drives moisture from the adsorbent, thus regenerating it ready for the drying of further compressed gas.
U.S. Pat. No. 5,681,370 describes a regenerative desiccant dryer system, in which two desiccant beds are employed. Each bed is operated alternately in a so-called "dry" mode, in which dry air is drawn from the bed, and a so-called "purge" mode, in which the bed is purged of moisture and the desiccant regenerated. Purging of the bed is achieved by heating the desiccant in order to remove the moisture retained by the desiccant during the operation in the dry mode. The apparatus of U.S. Pat. No. 5,681,370 comprises two desiccant beds in a housing. A heating element is provided in order to provide heat required to remove moisture and regenerate the desiccant.
U.S. Pat. No. 5,632,802 discloses an apparatus and method of regenerating adsorbers for drying. The apparatus and method for removing moisture from compressed air requires that moisture is removed from the air before the air is compressed. The apparatus to dry the pre-compressed air comprises a desiccant bed, through which the moisture-containing air is caused to pass before being compressed. Once the desiccant is saturated with adsorbed moisture, it is regenerated using heat. Air is drawn in through a filter and heated using a heating element. This heated air is then passed through the desiccant bed being regenerated. The action of the heated air on the desiccant is to desorb moisture by removing it from the pores of the desiccant. The desiccant beds are preferably stated to be in a radial configuration about a core member.
U.S. Pat. No. 5,037,458 relates to an apparatus for regenerating an adsorber in a gas drying plant. In the apparatus disclosed, two beds of adsorbent are used to dry moisture-containing compressed air. While in operation, one of the adsorbers is in the drying phase and dries the compressed gas, while the other of the adsorbers is in the regeneration phase. The regeneration of the adsorbent is carried out by taking a portion of the dried compressed air, mixing it with an amount of moisture-containing air and heating the resulting mixture by heat exchange with the moisture-containing compressed air. The thus heated mixture is passed through the bed of adsorbent to be regenerated in order to remove adsorbed moisture from the bed.
A similar method of drying a compressed gas to that described in U.S. Pat. No. 5,037,458 is disclosed in European Patent Application publication No. 0 799 635 A1. In this document, compressed gas leaving a compressor is dried by being passed through a bed of adsorbent. Adsorbent saturated with moisture is regenerated by using a portion of the dried compressed gas, which is first heated by utilizing heat generated by the compressor.
Prior art systems for drying compressed gas relying upon heating elements to regenerate the adsorbent, either directly or indirectly by means of the heating of a purge gas, require an additional supply of power, typically electricity. Systems relying upon the use of the dried compressed gas as the regenerating agent reduce the efficiency of the overall compressing and drying process, by diverting a portion of the dried compressed gas product away from the desired end use. Further inefficiencies arise once it is decided to treat the purge gas leaving the bed of adsorbent being regenerated and remove the moisture. Finally, systems relying upon the hot compressed gas as the source of heat for the regeneration stage depend upon the gas leaving the compressor being at optimum conditions for the regeneration of the adsorber. In many circumstances, this is not the case.
Accordingly, there is a need for an improved system for the compression and drying of a gas, for example air, which allows for an optimal use of the heat generated by the compressor to regenerate the adsorbent, while leaving the regeneration independent of the condition of the final compressed gas leaving the compressor. In addition, it would be advantageous if such a system would not have to rely upon an external supply of energy in order to provide or supplement the heat required for regeneration of the adsorbent.